This invention relates to an apparatus for controlling a T-die which extrudes a molten resin flow in the form of a sheet. More particularly, this invention relates to an apparatus which is the type of controlling a thickness of a molten resin flow.
Apparatus of the type above described have been already known and disclosed in Japanese Patent Prepublications (Kokai) Nos. 78,726 of 1983 and 184,832 of 1987.
In the manner which will later be described in connection with this invention, such an apparatus is for use in combination with the T-die.
In general, the T-die is used for extruding a sheet of a molten resin which has a width and a thickness. For this purpose, the T-die comprises a lip member having a pair of lip portions with a gap extended between the lip portions along a widthwise direction. The lip member is divisible into first through N-th zones along the widthwise direction. The sheet is extruded through the gap, in the form of the sheet. The T-die further comprises first through N-th driving devices which are mechanically coupled to the zones to individually drive the respective zones of the lip member with N-dimensional driving force vectors, respectively, and to partially and individually control the gap.
Such individual control of the zones of the lip structure brings about individual interference which might occur at adjacent zones of the lip member and which may be called mutual interaction. Due to such mutual interaction, the sheet extruded from the T-die often has a locally variable thickness, which degrades a quality of the sheet. Moreover, it is difficult to avoid influences of such mutual interaction.
Therefore, attempts have been tried to decrease the influences of the mutual interaction in such an apparatus. The apparatus comprises a comparing circuit which compares a plurality of partial thicknesses of the sheet measured at the respective zones with a predetermined desired thicknesses to produce thickness differences. A desired gap calculating circuit calculates N-dimensional desired displacement vectors (.DELTA.y) of the gap at every one of the zones from the thickness differences.
In addition, a command value processing circuit processes signals so as to individually control each of the driving devices by the use of proportional constants between the desired displacement vectors and each of the driving force vectors of the driving devices to decrease the influences of the mutual interaction.
With the above apparatus in the Japanese Patent Prepublication No. 78,726 of 1983, however, the influences of the mutual interaction could not be favorably decreased by the above-mentioned processing method. This is because the proportional constants inevitably include many uncertainties and result in.
Even with the processing way in the Japanese Patent Prepublication No. 184,832 of 1987 such that each of driving forces of the driving devices is repeatedly calculated to under the control output signals within a predetermined allowable range, it is necessary to solve linear equations defined by the proportional constants. This makes a calculation infavorably long and degrades reliability of the T-die.